Adhesives for laminating thermal print elements

ABSTRACT

A thermal print element comprising a support having thereon a layer containing a thermally-transferred dye image, the element having at least one layer of adhesive thereon comprising a linear, random coplyester of one or more aromatic dibasic acids and one or more aliphatic diols, modified with up to 30 mole % of one or more aliphatic dibasic acids. The copolyester has a melt viscosity of between about 1,000 and about 20,000 poise at 150° C. The adhesive is used to laminate a cover sheet to one or both surfaces of the thermal print element.

This application is a reissue application for U.S. Pat. No. 4,713,365issued on Dec. 15, 1987, based on U.S. Ser. No. 06/947,053, filed Dec.29, 1986..Iaddend.

This invention relates to the use of certain adhesives to laminate coversheets to thermal print elements for protective and security purposes.

In recent years, thermal transfer systems have been developed to obtainprints from pictures which have been generated electronically from acolor video camera. According to one way of obtaining such prints, andelectronic picture is first subjected to color separation by colorfilters. The respective color-separated images are then converted intoelectrical signals. These signals are then operated on to produce cyan,magenta and yellow electrical signals. These signals are thentransmitted to a thermal printer. To obtain the print, a cyan, magentaor yellow dye-donor element is placed face-to-face with a dye-receivingelement. The two are then inserted between a thermal printing head and aplaten roller. A line-type thermal printing head is used to apply heatfrom the back of the dye-donor sheet. The thermal printing head has manyheating elements and is heated up sequentially in response to the cyan,magenta and yellow signals. The process is then repeated for the othertwo colors. A color hard copy is thus obtained which corresponds to theoriginal picture viewed on a screen. Further details of this process andan apparatus for carrying it out are contained in U.S. Pat. No.4,621,271 by Brownstein entitled "Apparatus and Method for Controlling AThermal Printer Apparatus," issued Nov. 4, 1986, the disclosure of whichis hereby incorporated by reference.

The use of the above process to produce indentification (ID) cards isdescribed in copending application Ser. No. 947,052, of Sethi et al.,filed of even date herewith, entitled "High-Security Identification CardObtained By Thermal Dye Transfer". It would be desirable to provideprotection and additional tamper-proofing of such ID cards.

Heat-lamination of a cover sheet of polymeric film, such aspolycarbonate or a polyester such as poly(ethyleneterephthalate), overthe entire surface and extended sides of an ID card is an excellent wayto increase protection from surface abrasion and to minimize tampering.Attempts to remove the cover sheet ideally should result in itsdestruction to prevent alteration and reuse.

It would be desirable to provide an adhesive, meltable at a controlledtemperature, that adheres rapidly, firmly and uniformly, to the ID cardreceiver layer surface, card support stock, and a protective coversheet. It would also be desirable to provide an adhesive that would notadversely affect dye-stability nor alter definition of the thermaldye-transfer image or any auxiliary information on the card.

These and other objects are achieven in accordance with this inventionwhich comprises a thermal print element comprising a support havingthereon a layer containing a thermally-transferred dye image, theelement having at least one layer of adhesive thereon comprising alinear, random copolyester of one or more aromatic dibasic acids and oneor more aliphatic diols, modified with up to 30 mole % of one or morealiphatic dibasic acids, the copolyester having a melt viscosity ofbetween about 1,000 and about 20,000 poise at 150° C.

In a preferred embodiment of the invention, the aromatic dibasic acid isterephthalic acid, isophthalic acid, dipicolinic acid or2,2-bis(p-carboxyphenyl)propane. In another preferred embodiment, thealiphatic diol is ethyleneglycol, diethyleneglycol, triethyleneglycol,1,4-butanediol, 1,2-propanediol, 1,3-propanediol, 1,6-hexanediol,cyclohexanedimethanol, or 1,4-cyclohexanediol. In yet another preferredembodiment, the aliphatic dibasic acid is sebacic acid, glutaric acid,adipic acid, azelaic acid, or 1,4-cyclohexanedicarboxylic acid.

The adhesive layer may be solvent-coated to the cover sheet or may bethermally-applied as a self-supporting layer to either or both sides ofthe thermal print element or the cover sheet. Any thickness of adhesivemay be used provided it provides a secure bond to the cover sheet. Ingeneral, good results hav been obtained using the thickness of fromabout 5 to about 100 μm.

The layer containing the dye image employed in the invention maycomprise, for example, a polycarbonate, a polyurethane, a polyester,polyvinyl chloride, poly(styrene-co-acrylonitrile), poly(caprolactone)or mixtures thereof. The dye image-receiving layer may be present in anyamount which is effective for the intended purpose. In general, goodresults have been obtained at a concentration of from about 1 to about 5g/m².

In a preferred embodiment, a polycarbonate layer containing the dyeimage is used which has a number average molecular weight of at leastabout 25,000. The term "polycarbonate" as used herein means a polyesterof carbonic acid and glycol or a divalent phenol. Examples of suchglycols or divalent phenols are p-xylene glycol,2,2-bis(4-oxyphenyl)propane, bis(4-oxyphenyl)methane,1,1-bis(4-oxyphenyl)ethane, 1,1-bis-oxyphenyl)butane,1,1-bis(oxyphenyl)cyclohexane, 2,2-bis(oxyphenyl)butane, etc.

In an especially preferred embodiment of the invention, theabove-described polycarbonate is a bisphenol A polycarbonate. In anotherpreferred embodiment of the invention, the bisphenol A polycarbonatecomprises recurring units having the formula: ##STR1## wherein n is fromabout 100 to about 500.

Examples of such polycarbonates include: General Electric Lexan®Polycarbonate Resin #ML-4735 (Number average molecular weight app.36,000), and Bayer AG, Makrolon #5705® (Number average molecular weightapp. 58,000).

The polycarbonate employed in the layer containing the dye image may bepresent in any amount which is effective for the intended purpose. Ingeneral, good results have been obtained at a total concentration offrom about 1 to about 5 g/m².

As noted above, the adhesives of the invention are used to laminate acover sheet to one or both surfaces of the thermal print element. Therecan be used as the cover sheet, for example, various polymeric materialssuch as poly(ethylene terephthalate), polycarbonate, polystyrene,polyethylene, cellulose acetate, poly(vinyl alcohol-co-acetal), etc.

Specific copolyesters useful in the invention include the followingmaterials:

Compound (1): a random copolyester formed from 1,4-butanediol;1,6-hexanediol; and terephthalic acid (mole ratio of diols: 80% C₆, 20%C₄)

Compound (2): a random copolyester formed from 1,4-butanediol;1,6-hexanediol; terephthalic acid; and isophthalic acid (mole ratio ofdiols: 65% C₆, 35% C₄ ; mole ratio of acids: 90% terephthalic, 10%isophthalic)

Compound (3): a random copolyester formed from diethyleneglycol;1,4-butanediol; terephthalic acid; and glutaric acid (mole ratio ofdiols: 55% C₄, 45% gylcol; mole ratio of acids: 70% terephthalic, 30%glutaric)

Compound (4): a blend of a random copolyester formed from (3) and arandom copolyester formed from cyclohexanedimethanol; ethyleneglycol;and terephthalic acid (mole ratio of diols: 69% C₂, 31%cyclohexanedimethanol)

Compound (5): a random copolyester formed from 1,4-butanediol;1,6-hexanediol; terephthalic acid; and isophthalic acid (mole ratio ofdiols: 80% C₆, 20% C₄ ; mole ratio of acids: 80% terephthalic, 20%isophthalic)

Compound (6): a random copolyester formed from 1,4-butanediol; sebacicacid; terephthalic acid; and isophthalic acid (mole ratio of acids: 15%sebacic, 35% terephthalic, 50% isophthalic) sold commercially as Bostik7962 (Bostik Chemical Group, Emhart Corp.)

The support for the thermal print element of the invention may be atransparent film such as a poly(ether sulfone), a polyimide, a celluloseester such as cellulose acetate, a poly(vinyl alcohol-co-acetal) or apoly(ethylene terephthalate). The support may also be reflective such asbaryta-coated paper, white polyester (polyester with white pigmentincorporated therein), an ivory paper, a condenser paper or a syntheticpaper such as duPont Tyvek®. In a preferred embodiment, polyester with awhite pigment incorporated therein is employed. It may be employed atany thickness desired, usually from about 50 μm to about 1000 μm.

A dye-donor element that is used with the thermal print element of theinvention comprises a support having thereon a dye layer. Any dye can beused in such a layer provided it is transferable to the dyeimage-receiving layer of the dye-receiving element by the action of heatto provide the thermal print. Especially good results have been obtainedwith sublimable dyes. Examples of sublimable dyes include anthraquinonedyes, e.g., Sumikalon Violet RS® (product of Sumitomo Chemical Co.,Ltd.) Dianix Fast Violet 3R-FS® (product of Mitsubishi ChemicalIndustries, Ltd.), and Kayalon Polyol Brilliant Blue N-BGM® and KSTBlack 146® (products of Nippon Kayaku Co., Ltd.); azo dyes such asKayalon Polyol Brilliant Blue BM®, Kayalon Polyol Dark Blue 2BM®, andKST KR® (products of Nippon Kayaku Co., Ltd.), Sumickaron Diazo Black5G® (product of Sumitomo Chemical Co., Ltd.), and Miktazol Black 5GH®(product of Mitsui Toatsu Chemicals, Inc.); direct dyes such as DirectDark Green B® (product of Mitsubishi Chemical Industries, Ltd.) andDirect Brown M® and Direct Fast Black D® (products of Nippin Kayaku Co.Ltd.); acid dyes such as Kayanol Milling Cyanine 5R® (product of NipponKayaku Co. Ltd.); basic dyes such as Sumicacryl Blue 6G® (product ofSumitomo Chemical Co., Ltd.), and Aizen Malachite Green® (product ofHodogaya Chemical Co., Ltd.); ##STR2## or any of the dyes disclosed inU.S. Pat. No. 4,541,830, the disclosure of which is hereby incorporatedby reference. The above dyes may be employed singly or in combination toobtain a monochrome. The dyes may be used at a coverage of from about0.05 to about 1 g/m² and are preferably hydrophobic.

The dye in the dye-donor element is dispersed in a polymeric binder suchas a cellulose derivative, e.g., cellulose acetate hydrogen phthalate,cellulose acetate, cellulose acetate propionate, cellulose acetatebutyrate, cellulose triacetate; a polycarbonate;poly(styrene-co-acrylonitrile), a poly(sulfone) or a poly(phenyleneoxide). The binder may be used at a coverage of from about 0.1 to about5 g/m².

The dye layer of the dye-donor element may be coated on the support orprinted thereon by a printing technique such as a gravure process.

Any material can be used as the support for the dye-donor elementprovided it is dimensionally stable and can withstand the heat of thethermal printing heads. Such materials include polyesters such aspoly(ethylene terephthalate); polyamides; polycarbonates; glassinepaper; condenser paper; cellulose esters such as cellulose acetate;fluorine polymers such as polyvinylidene fluoride orpoly(tetrafluoroethylene-co-hexafluoropropylene); polyethers such aspolyoxymethylene; polyacetals; polyolefins such as polystyrene,polyethylene, polypropylene or methylpentane polymers; and polyimidessuch as polyimide-amides and polyether-imides. The support generally hasa thickness of from about 2 to about 30 μm. It may also be coated with asubbing layer, if desired.

A dye-barrier layer comprising a hydrophilic polymer may also beemployed in the dye-donor element between its support and the dye layerwhich provides improved dye transfer densities. Such dye-barrier layermaterials include those described and claimed in application Ser. No.813,294 entitled "Dye-barrier Layer for Dye-Donor Element Used inThermal Dye Transfer" by Vanier et al, filed Dec. 24, 1985.

The reverse side of the dye-donor element may be coated with a slippinglayer to prevent the printing head from sticking to the dye-donorelement. Such a slipping layer would comprise a lubricating materialsuch as a surface active agent, a liquid lubricant, a solid lubricant ormixtures thereof, with or without a polymeric binder. Preferredlubricating materials include oils or semi-crystalline organic solidsthat melt below 100° C. such as poly(vinyl sterate), beeswax,perfluorinated alkyl ester polyethers, poly(caprolactone), carbowax orpoly(ethylkene glycols). Suitable polymeric binders for the slippinglayer include poly(vinyl alcohol-co-butyral), poly(vinylalcohol-co-acetal), poly(styrene), poly(-vinyl acetate), celluloseacetate butyrate, cellulose acetate or ethyl cellulose.

The amount of the lubricating material to be used in the slipping layerdepends largely on the type of lubricating material, but is generally inthe range of about 0.001 to about 2 g/m². If a polymeric binder isemployed, the lubricating material is present in the range of 0.1 to 50weight %, preferably 0.5 to 40, of the polymeric binder employed.

As noted above, dye-donor elements are used to form a dye transfer imagein the thermal print. Such a process comprises imagewise-heating adye-donor element and transferring a dye image to a dye-receivingelement as described above to form the dye transfer image in a thermalprint element.

The dye-donor element employed with the thermal print elements of theinvention may be used in sheet form or in a continuous roll or ribbon.If a continuous roll or ribbon in employed, it may have only one dyethereon or may have alternating areas of different dyes such as cyan,magenta, yellow, black, etc., as disclosed in U.S. Pat. No. 4,541,830.

In a preferred embodiment of the invention, a dye-donor element isemployed which comprises a poly(ethylene terephthalate) support coatedwith sequential repeating areas of cyan, magenta and yellow dye, and theabove process steps are sequentially performed for each color to obtaina three-color dye transfer image. Of course, when the process is onlyperformed for a single color, then a monochrome dye transfer image isobtained.

Thermal printing heads which can be used to transfer dye from thedye-donor elements employed in the invention are available commercially.There can be employed, for example, a Fujitsu Thermal Head (FTP-040MCS001), a TDK Thermal Head F415 HH7-1089 or a Rohm Thermal Head KE2008-F3.

The following examples are provided to illustrate the invention.

EXAMPLE 1 Adhesion Test

A three-sheet composite was prepared in the following manner. A 175 μmthick thermoformed sheet of General Electric Lexan 131 (a bisphenol Apolycarbonate) was laminated to a 75 μm thick thermal formed or solventcast sheet of the polyester adhesive as indicated in the Table bypassage through a Laminex Co. PA-KIII-E laminating device at 150° C.

A second sheet of the same polycarbonate material was laminated to theadhesive side of the above composite by passage through the laminatingdevice at 175° C. A siliconized paper tab was inserted on a portion ofthe composite for the second lamination. Both laminations were made witha paper carrier release liner to avoid fusing to the laminator.

The composite was allowed to stand at room temperature for a minimum of14 hours before an adhesion measurement. A test of the adhesive bond ofthe polycarbonate to the polyester adhesive was then run using anInstron Universal Testing Machine Model TM-1122. A one-inch wide samplewas cut from the composite and formed into a T-peel joint for testing ata peel rate of 5 inches/min. Measured peel forces above 1000 g/in aredesirable. The results obtained are tabulated in the Table.

Viscosities of each invention and control polyester were evaluated by astandard melt-shear viscosity test at a shear rate of 1/sec at 150° C.The following results were obtained:

                  TABLE                                                           ______________________________________                                        Adhesive    Viscosity (poise)                                                                          Adhesion (g/in)                                      ______________________________________                                        Cmpd.   1       7,800        2500                                             Cmpd.   2       7,500        1750                                             Cmpd.   3       6,900        3500                                             Cmpd.   4       10,500       5000                                             Cmpd.   5       6,000        6000                                             Cmpd.   6       4,000        5250                                             Control 1       150000       100                                              Control 2       >>150000     17                                               ______________________________________                                         Control 1: a random copolyester formed from cyclohexanedimethanol;            ethyleneglycol; and terephthalic acid (mole ratio of diols: 69% C.sub.2,      31% cyclohexanedimethanol)                                                    Control 2: a random copolyester formed from cyclohexanedimethanol;            terephthalic acid; and isophthalic acid (mole ratio of acids: 83%             terephthalic, 17% isophthalic)                                           

The above results indicate that the copolyesters of the invention havemuch superior adhesion than the comparison copolyesters.

EXAMPLE 2 Lamination of ID Card

(A) A yellow dye-donor element was prepared by coating the followinglayers in the order recited on a 6 μm poly(ethylene terephthalate)support:

(1) Dye-barrier layer of gelatin nitrate (gelatin, cellulose nitrate andsalicylic acid in approximately 20:5:2 weight ratio in a solvent ofacetone, methanol and water) (0.17 g/m²),

(2) Dye layer containing the following yellow dye (0.39 g/m²) incellulose acetate (40% acetyl) (0.38 g/m²) coated from a butanone,acetone and cyclohexanone (14:8:1) solvent mixture: ##STR3## On the backside of the element, a slipping layer of poly(vinyl stearate) (0.3 g/m²)in polyvinylbutyral (Butvar-76® Monsanto) (0.45 g/m²) was coated fromtetrahydrofuran solvent.

(B) A magenta dye-donor element was prepared similar to (A) except thatthe dye layer (2) comprised the following magenta dye (0.22 g/m²) incellulose acetate hydrogen phthalate (0.38 g/m²) coated from a butanone,acetone and cyclohexanone (14:4:1) solvent mixture: ##STR4##

(C) A cyan dye-donor element was prepared similar to (A) except that thedye layer (2) comprised the following cyan dye (0.37 g/m²) in celluloseacetate hydrogen phthalate (0.42 g/m²) coated from a butanone, acetoneand cyclohexanone (14:4:1) solvent mixture: ##STR5##

A dye-receiving element was prepared by coating a solution of Makrolon5705® (a bisphenol A polycarbonate resin from Bayer AG) from a methylenechloride and trichloroethylene solvent mixture at a coverage of 2.9 g/m²on top of an ICI Melinex® "White Polyester" reflective support:

The dye side of each dye-donor element strip 3 inches (75 mm) wide wasplaced in contact with the dye image-receiving layer side of the printedcard stock. The assemblage was fastened in the jaws of a stepper motordriven pulling device. The assemblage was laid on top of a 0.55 (14 mm)diameter rubber roller and a Fujitsu Thermal Head (FTP-040MCS001) andwas pressed with a spring at a force of 3.5 pounds (1.6 kg) against thedye-donor element side of the assemblage pushing it against the rubberroller.

The imaging electronics were activated causing the pulling device todraw the assemblage between the printing head and roller at 0.123inches/sec (3.1 mm/sec). Coincidentally, the resistive elements in thethermal print head were heated at controlled times from 0 to 4.5 msec togenerate a continuous tone personalized test image. The voltage suppliedto the print head was approximately 19 v representing approximately 1.75watts/dot. A multicolor ID card was thus obtained.

The above-obtained ID card was then laminated for protection in thefollowing manner. Cover sheets of 175 μm thick polycarbonate were firstlaminated to a 75 μm thick thermal-formed or solvent-cast adhesivesheet, the laminated to the ID card described above. A secondadhesive-laminated cover sheet was subsequently laminated to theback-side of the ID card. This provided protection for front and backsides of the printed card.

A forced peel of the double-laminated composite using the inventioncopolyester adhesives stripped the polycarbonate dye-receiver layercontaining the thermally-transferred dye image off the white polyestersupport of the thermal print. This indicates that a "secure" laminationwas obtained, i.e., one that could not be tampered with withoutdetection.

However, peeling of the double-laminated composite using the controlcopolyester adhesives resulted merely in easy delamination at theadhesive layer-receiver layer interface of the adhesive layer-coversheet interface. Because of this weak adhesion, durability and securityof this composite was unsatisfactory.

Visual observation of image smear was also made on the laminated cards.None of the invention adhesive polymers produced any noticeable smear.In addition, none of the invention adhesive polymers had any adverseaffect on dye-light stability.

The invention has been described in detail with particular reference tothe preferred embodiments thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

What is claimed is:
 1. A thermal print element comprising a supporthaving thereon a layer containing a thermally-transferred dye image,said element having at least one layer of adhesive thereon comprising alinear, random copolyester of one or more aromatic dibasic acids and oneor more aliphatic diols, modified with up to .[.30.]. .Iadd.15.Iaddend.mole % of one or more aliphatic dibasic acids, said copolyesterhaving a melt viscosity of between about 1,000 and about 20,000 poise at150° C.
 2. The element of claim 1, wherein sai aromatic dibasic acid isterephthalic acid, isophthalic acid, dipicolinic acid or2,2-bis(p-carboxyphenyl)propane.
 3. The element of claim 1 wherein saidaliphatic diol is ethyleneglycol, diethyleneglycol, triethyleneglycol,1,4-butanediol, 1,2-propanediol, 1,3-propanediol, 1,6-hexanediol,cyclohexanedimethanol, or 1,4-cyclohexanediol.
 4. The element of claim 1wherein said aliphatic dibasic acid is sebacic acid, glutaric acid,adipic acid, azelaic acid, or 1,4-cyclohexanedicarboxylic acid.
 5. Theelement of claim 1 wherein said layer containing said dye image is apolycarbonate having a number average molecular weight of at least about25,000.
 6. The element of claim 5 wherein said polycarbonate is abisphenol A polycarbonate comprising recurring units having the formula:##STR6## wherein n is from about 100 to about
 500. 7. The element ofclaim 1 wherein said support is poly(ethylene terephthalate) having awhite pigment incorporated therein.
 8. The element of claim 1 which hasa cover sheet laminated to at least one outer surface by said adhesive.9. The element of claim 8 wherein said cover sheet is poly(ethyleneterephthalate).
 10. The element of claim 8 wherein said cover sheet ispolycarbonate.
 11. The element of claim 1 which has a poly(ethyleneterphthalate cover sheet laminated to both surfaces thereof by saidadhesive.
 12. The element of claim 1 which has a polycarbonate coversheet laminated to both surfaces thereof by said adhesive.
 13. Theelement of claim 1 wherein said copolyester is formed from1,4-butanediol; 1,6-hexanediol; and terephthalic acid.
 14. The elementof claim 1 wherein said copolyester is formed from 1,4-butanediol;1,6-hexanediol; terephthalic acid; and isophthalic acid.
 15. The elementof claim 1 wherein said copolyester is formed from diethyleneglycol;1,4-butanediol; terephthalic acid; and glutaric acid.
 16. The element ofclaim 1 wherein said copolyester is formed from a blend of a randomcopolyester formed from diethyleneglycol; 1,4-butanediol; terephthalicacid; and glutaric acid; and a random copolyester formed fromcyclohexanedimethanol; ethyleneglycol; and terephthalic acid.
 17. Theelement of claim 1 wherein said copolyester is formed from1,4-butanediol; sebacic acid; terephthalic acid; and isophthalic acid.